Method of forming a photoconductive target electrode



Feb. 26, 1957 A. D. COPE ETAL 2,733,117

METHOD OF FORMING A PHOTOCONDUCTIVE TARGET ELECTRODE Filed Ma '14, 1952 1 INVENTOR. /4PPLETON D. 601 5 ROBE/PT fit 600mm,!

ORNEY United States Patent o.

METHOD OF FORMING A PHOTOCONDUCTIVE TARGET ELECTRODE ApplicationMay 14, 1952 Serial No. 287,666 7 Claims. (Cl. 316-8) This invention relates to elements sensitive to radiant energy and particularly to a method of preparing such an element having a layer of photoelectric material.

In the electronic arts, many devices such as camera pickup tubes, phototubes' and the like, utilize radiant energy sensitive elements such as targets or photocathodes or other elements having layers of photoelectric material which may be deposited by an evaporation process. Selenium, antimony trisulfide, antimony and caesium are examples of photoelectric materials which are used in such applications.

The principles of this invention are particularly applicable to electron tubes utilizing that class of photoelectric materials known as photoconductors including antimony trisulfide, and the like. One such electron tube of the pickup type is described in an article in the RCA Review of September 1951 on pages 307 to 310 and in v a copending- U. S. patent application of S. V. Forgue, Serial No. 198,130, filed November 29, 1950; This camera tube includes an elongated tubular envelope containing an electron gun at one end and a mesh screen electrode closely spaced adjacent to a target assembly contained in the other end'of the. envelope. The electron gun is a conventional type used to .form an electron beam and direct the beam toward the target assembly. The-target assembly comprises a film of light transparent electrically conductive material forming a signal plate on the glass face plate of the envelope and a signal lead connected thereto. A layer of photoconductive material is deposited upon the electrically conductive film to complete the target. The target assembly and the gun are so arranged within the envelope that, with proper'potentials and deflection, an electron beam from the gun scans. the photoconductive film or photolayer.

The photoconductive material used in such a tube is an electrical insulator in the dark, but it becomes electrically conductive under the influence of light. The conductivity increases with the amount of lightafifecting the material, and is limited to the immediate area under the influence of the light.

In operation of this tube, the transparent,electrically conductive film or signal plate is connected to a voltage source by means of the signal lead or signal ring. This arrangement establishes the desired operating potential difference across the photoconductive layer. This difference is established with respect to the cathode which is. at ground potential. 'The electron beam scans the particular area, and causesthe correspondingportion.of-

the scannecl surface of the photoconductive film to give up electrons and move a volt or so closer in potential to the conductive film. The next time the electron beam scans this area, it restores to cathode potential the area from which electrons have leaked under the light-induced conductivity. This return to cathode potential restores the potential difference across the photoconductive layer and causes an electron current to the signal plate from the source of potential to which it is electrically connected. This electron current flows through an output resistor andprovides the video signal output from the tube. In this type of operation, the screen electrode adjacent the photoconductive film provides a uniform decelerating field for the electron beam as it approaches the photoconductive surface. a

Some of the qualities of photoconductive materials which must be considered in determining their desirability as films in pickup tube target assemblies are: sensitivity, resistivity in the dark, and lag.

Sensitivity has reference to the ability of the material to become conductive under the influence of light. It is measured in micro-amperes of video current output per lumen of light on the target.

Resistivity in the dark has reference to that quality of photoconductive material which enables it to store an electrical charge in a given spot without leakage from front to back surface as long as there is no light on the target.

By lag is meant the rate of response of the photoconductor'to changes in light, i. e. the ability of the photolayer to erase a signal in a given period of time without showing a shadow or trail of light. The problems arising from lag become acute when a light-colored moving object is televised against a dark background.

The electrical characteristics, such as sensitivity, resistivity and the like, of photoconductive films such as films of antimony trisulfide, and the like, depend, among other things, on the physical characteristics of these films. For example, if the film has been formed by evaporation in a vacuum and if it has been heated 'excessively, the film becomes a smooth solid homogeneous layer in'which the chemical material is in or near the metallic state. desirable. However, it also has high lag and low resistivity. This type of film is unsuitable for use in a camera tube. However, if the photoconductive layer is deposited in a porous, spongy condition, as by evaporation in a poor vacuum, such a layer has higher resistivity and lower lag and even though the sensitivity is reduced, the film is more suitable for the desired use. Since the photoconductive material cannot be made more porous once it has been deposited on the target area, the problem is to provide a method of preparing a photoconductive film having an optimum balance between the various:

electrical characteristics. 1

ductive film in a light sensitive device.

Another object is to provide a method of preparing a photoconductive film having high sensitivity.

A further object is to provide a method of preparing.

an improved photoconductive film havinglow lag.

Another objectis to provide a method of preparinga photoconductive film. characterized by an optimum bal;

ance' between sensitivity, resistivity and lag.

In general the purposes and objects of this invention are accomplished by evaporating a porous, spongy layer Such a film has high sensitivity which is spear 17 it until an optimum balance is achieved between the sensitivity; the resistivity and the lag of the film.

The principles of the invention are described with refcrence to the drawing whieh shows a tube of the kind described above having a photoconductive film prepared according to the method of the invention.

The tube includes an envelope or bulb which is provided with an electron gun 12 including a suitable number of electrodes 14 and a final tubular accelerating electrode 16 having a mesh screen 18 mounted at one end thereof. A layer 20 of transparent conductive material, for example a layer of the type which may be formed by pyrolitic deposition from a mixture of air or oxygen and the vapors of tin (stannic) chloride and methanol, is coated in the inner surface of the face plate 22 of the envelope.

A conductive ring 24 is embedded in the wall of the envelope adjacent to the face plate and in contact with the conductive coating 20. The conductive ring 24 constitutes a signal lead or ring and may be connected in According to a standard assembly procedure, after the aforementioned components are mounted in the envelope 10, an evaporator is charged with a quantity of antimony trisulfide, or other chemical to be evaporated. Any suitable evaporator may be used for this process and one such device is shown in a copending application of B. H. Vine, Serial No. 279,883 filed April 1, 1952 now Patent No. 2,733,115, issued January 31, 1956 and assigned to the assignee of this application. The evaporator show-n therein includes a chemical carrying receptacle carried by a support means 36 which comprises a pair of conductive arcuate members connected by a conductive end piece 38. A pair of support rods 40 are connected to the support means 36 and are maintained in spaced relationship by a ceramic member 42. If heating is to be eiieeted directly by electrical current flow through the evaporator, the rods 40 are retained in spaced relationship and each is provided with a conductive wing 44. These conductive members 4-4 are adapted to be connected to a source of heating voltage through contacts placed in the side tube 28. frequency heating coil, the current path formed by the support means 36 and support rods 40 is completed by a suitable connector between the rods 40 in place of the conductive wings 44.

The charge carrying evaporator is inserted into the side tube 28 and the open end thereof is sealed. Next, the tube envelope 10 is connected to the usual exhaust device by means of the exhaust appendage 26 and the envelope is pumped out. With the evaporator completely retracted in the side tube to protect the chemical charge from heat, the bulb is baked in order to degas the envelope and its components. Baking may be effected by any suitable means, for example in a split oven which comprises two arcuate oven portions which are positioned around the envelope. The tube is baked at a temperature in the rangeof 400 C. to 450 C. for approximately one hour. Next, theenvelope is cooled and getters 32, one of which is shown, are degassed, for example, by passing an electric current therethrough. By this procedure, gases are removed from the getter support and from the binder matrix in which the getter material is embedded. Other processing steps may be included as required.

After the bulb 10 and its included components have been thus processed, the evaporator 30 is moved into the If the evaporator is to be heated by a high 4 envelope into charge-evaporating position by means of a magnet 31 or other suitable means. In this position of the evaporator, the photoconductive charge carried thereby is disposed within the envelope in position to be readily evaporated and deposited onto the transparent conductive layer 20. The exhaust apparatus is disconnected from the exhaust tube 26 and a gas which will not react with the photoconductive chemical, for example, air, argon, helium, or the like is introduced into the tube in a quantity sumcient to establish a gas pressure of one to five miilimeters of mercury.

The evaporator 30 is then heated to vaporize the charge. It may be heated inductively, by means of a high frequency induction heating coil, or directly, by current flow from a source of alternating or direct current as shown in the Vine patent previously mentioned. As the evapora tor is heated, the chemical isheated and vaporized. By this procedure, the photoconductive material is evaporated and deposited on the conductive film as a porous, .flutfy layer 29. The desired thickness of the photoconductive layer may be determined in. many ways. For example a qualitative light transmission method may be used. Other methods are described in a copcnding application of Forgue and Goodrich, Serial Number 229,428, filed June l, 195] now Patent No. 2,745,032, issued May 8, 19.56, and assigned to the assignee of this application. One such method is to count interference fringes resulting from illumination of the film by monochromatic light. An' other method is to compare the evaporated film with a standard film by using a suitable optical test. After the evaporation has ben completed, the evaporator 30 is retraeted into the side tube 28.

During the evaporation procedure, some of the antimony trisulfide often fails to pass through the mesh screen 18 and is retained thereby. Such material, if present on the screen during tube operation, upsets the electron optics of the tube. To solve this problem, a high frequency heating coil 34 or some other suitable means is positioned around the envelope 10 in the vicinity of the target assembly and the tubular electrode 16 and screen 18 are thereby heated. This heating re-evaporates the photoconductive material onto the conductive layer. This process is describedin detail in a copending application of F. S. Veith, Serial Number 279,746, filed April l, 1952 now abandoned.

According to the next step in the procedure the con nection between the bulb and the exhaust apparatus is restored and the bulb is exhausted to remove the gas atmosphere.

According to the invention, the photoconductive film 29 is then baked to consolidate the porous mass of chemical and to achieve the desired balance between resistivity, sensitivity and lag. The baking procedure is effected by any suitable means. an oven in which the tube may be inserted while still connected to the exhaust apparatus. Baking may also be carried out by radiant energy from a lamp directed at the target assembly.

According to the invention when an oven is employed for baking, the photoconductive film is baked [or 15 to 30 minutes at a ter .eratnre in the range of C. to C. This temperature and time of baking provide optimum balance between sensitivity, resistivity and lag. The sensitivity at the point of optimum balance is determined by shining a known quantity of light on the. face plate of a completed tube and measuring the signal output current. This output current is compared withthe output current of calibrated phototube for the same amount of light. The lag of a photoelectric film at this point is measured qualitatively by shining a burst of light on the target and scanning the illuminated area. The resulting signal output is viewed on an oscilloscope and the number of scans which maybe made before the signal dies out is an indication of the lag. Of course, other suitable methodsmay be employed for measuring the sensitivity and-1:13..

The most satisfactory heating means is Such a test is described in detail in an article by R. W. Smith in the September 1951 RCA Review.

Next, the side appendage 28 is tipped off and removed along with the evaporator 30. During this operation, the photoconductive film 31 is protected from heat and undesirable chemical changes. Gne means for providing such protection is a cap of wet asbestos and Dry ice covering the face plate and target a: embly area. The tube envelope is then exhausted until a vacuum on the order of 1 10- millimeters of mercury is established. The exhaust tubulation Ed is then tipped off and the tube is processed to completion by other processing steps as needed.

An alternative and suitable method of assembling the tube is described in the Forgue and Goodrich patent re ferred to above. processing the envelope, then evaporating the photoconductive film onto the coated face plate and finally mounting the electron gun Within the envelope. Of course, other necessary processing steps are also included. By this method, the side appendage 28 is not required and, generally, a tungsten wire crucible is positioned within the tube envelope as the evaporator. After the tube is thus assembled, the photoelectric film is baked as described above and the tube is further processed to completion.

What is claimed is:

1. The method of forming a photoconductive film in an electron tube envelope comprising introducing a gas into said envelope to a pressure of one to five millimeters of mercury, introducing a quantity of antimony trisulfide into said envelope, evaporating said antimony trisulfide in the presence of said gas onto a predetermined area within said envelope as a porous layer, removing said gas from said envelope, baking said photoconductive layer for to 30 minutes at a temperature in the range of 100 C. to 150 C. to consolidate the particles of said layer and to achieve an optimum balance between the sensitivity, resistivity and lag of said layer, and removing any remaining gas from said envelope.

2. The method of forming a photoconductive target electrode in an electron discharge device having an en velope and comprising forming a conductive transparent base within said envelope, introducing gas within said envelope at a pressure between one to five millimeters of mercury, introducing a quantity of antimony trisulfide within said envelope, applying heat to said antimony trisulfide in the presence of said gas to evaporate the same and deposit said antimony trisulfide on said conductive transparent base as a porous layer, removing said .gas from said envelope, baking said porous layer for 15 to 30 minutes at a temperature in the range of 100 C. to 150 C. to consolidate the particles of said layer and to achieve an optimum balance between the sensitivity, resistivity and lag of said layer, and removing any remaining gas from said envelope and establishing a high vacuum therein.

This method includes basically first 3. The method of forming a photoconductive target electrode in an electron discharge device having an envelope and comprising forming a conductive transparent base within said envelope, introducing a gas of the group comprising air, helium and argon within said envelope at a pressure between one to five millimeters of mercury, introducing a quantity of antimony trisulfide Within said envelope, applying heat to said antimony trisulfide in the presence of said gas to evaporate the same and deposit said vaporized material on said conductive transparent base as a porous layer, removing said gas from said envelope, baking said porous antimony trisulfide for 15 to 30 minutes at a temperature in the range of C. to C. to consolidate the particles of said layer and to achieve an optimum balance between the sensitivity, resistivity and lag of said layer and removing any remaining gas from said envelope and establishing a high vacuum therein.

4. The method of forming a photoconductive film on a support comprising evaporating antimony tri-sulphide in the presence of an inert gas maintained at low pressure to form a porous photoconductive film of antimony tri-sulphide on said support, and heating said porous film to consolidate the particles thereof and reduce the porosity of said film.

5. The method of forming a photoconductive layer on a support comprising evaporating antimony tri-sulphide in the presence of an inert gas maintained at a pressure of the order 'of one to five millimeters of mercury to form a porous photoconductive layer of antimony trisulphide on said support, and baking said porous layer for 15 to 30 minutes 'at a temperature substantially in the range of 100 C. to 150 C. to reduce its porosity and achieve an optimum balance between sensitivity, resistivity, and lag of said layer.

6. The method of forming a photoconductive layer on a target support comprising evaporating antimony trisulphide in the presence of an inert gas maintained at a pressure of the order of one to five millimeters of mercury to form a porous photoconductive layer of said antimony tri-sulfide on said support, and baking said layer in a relatively high vacuum at a temperature substantially within the range of 100 C. to 150 C. to achieve an optimum balance between the sensitivity, resistivity and lag of said layer.

7. The invention as in claim 6 wherein said baking is performed for 15 to 30 minutes.

References Cited in the file of this patent UNITED STATES PATENTS 1,623,323 Van Voorhis Apr. 5, 1927 2,189,986 Hickok Feb. 13, 1940 2,431,401 Janes Nov. 25, 1947 2,667,600 Golf Jan. 26, 1954 

1. THE METHOD OF FORMING A PHOTOCONDUCTIVE FILM IN AN ELECTRON TUBE ENVELOPE COMPRISING INTRODUCING A GAS INTO SAID ENVELOPE TO A PRESSURE OF ONE TO FIVE MILLIMETERS OF MERCURY, INTRODUCING A QUANTITY OF ANTIMONY TRISULFIDE INTO SAID ENVELOPE, EVAPORATING SAID ANTIMONY TRISULFIDE IN THE PRESENCE OF SAID GAS ONTO A PREDETERMINED AREA WITHIN SAID ENVELOPE AS A POROUS LAYER, REMOVING SAID GAS FROM SAID ENVELOPE, BAKING SAID PHOTOCONDUCTIVE LAYER FOR 15 TO 30 MINUTES AT A TEMPERATURE IN THE RANGE OF 100*C. TO 150*C. TO CONSOLIDATE THE PARTICLES OF SAID LAYER AND TO ACHIEVE AN OPTIMUM BALANCE 